Jacket tube made of synthetically produced quartz glass and optical fibres produced using said jacket tube

ABSTRACT

Jacket tubes of synthetically produced quartz glass as a semi-finished product for producing an outer cladding glass layer of an optical fiber are generally known. The invention relates to an improvement of a jacket tube in terms of inexpensive producibility and of suitability as a semi-finished product for optical fibers having a low optical attenuation. According to the invention this object is achieved by a jacket tube in which the quartz glass has a content of metastable OH groups of less than 0.05 wt ppm and a content of anneal-stable OH groups of less than 0.05 wt ppm.

BACKROUND OF THE INVENTION

The present invention relates to a jacket tube made of syntheticallyproduced quartz glass as a semi-finished product for producing an outercladding glass layer of an optical fiber.

Furthermore, the present invention relates to an optical fiber producedby using the jacket tube, comprising a core with a diameter d_(K) and afirst cladding region cladding the core and having an outer diameterd_(M), and a second cladding region cladding the first cladding region,the ratio of d_(M)/d_(K) being at least 2.5.

Optical fiber preforms for commercial applications are substantiallyproduced according to the known OVD (outside vapor deposition) MCVD(modified chemical vapor deposition), PCVD (plasma-induced chemicalvapor deposition) and VAD (vapor axial deposition) methods. In thesemethods, a core rod is first produced, which substantially forms thecore and the optically effective part of the cladding of the lateroptical fiber. The optically effective cladding region of the opticalfiber will be called “inner cladding” in the following.

Typical diameter ratios of core rod to core diameter are between 2 and5. Said diameter ratio is known as the so-called “dm/d_(K) ratio”, whered_(M) is the diameter of the core rod and d_(K) the diameter of thecore. Since commercially used single-mode optical fibers have typicalcore diameters of about 8 μm to 9 μm and a fiber diameter of 125 μm,further quartz glass must be applied to the core rod to achieve saidgeometrical ratios. Said further quartz glass forms an “outer cladding”of the fiber and is also called “jacket”.

At a typical d_(M)/d_(K) ratio of 4, the core rod contributes barely 10%to the whole fiber cross-section; the remaining 90% derive from thejacket material. For optimizing the costs of preform production, thecosts for producing and applying the jacket material are therefore ofcentral importance. So far it has generally been believed that thequality of the jacket material is of essential importance to themechanical strength of the later optical fiber whereas the influence onthe optical properties has so far been considered to be small.

The jacket material is normally provided in the form of an overcladdingtube of quartz glass or of porous SiO₂ soot material that prior to fiberdrawing or during fiber drawing is collapsed onto the optical cladding.

A method for producing a quartz glass preform for so-called single-modefibers using a jacket tube of the generic type and a fiber of theabove-mentioned kind are known from U.S. Pat. No. 4,675,040. In a firststep of the method, a core rod is produced by cladding a rod of coreglass with a cladding tube and fusing the same. The core of the core rodproduced in this way has a diameter of 8 mm and is clad with an innercladding having a smaller refractive index, the difference of therefractive indices being indicated as Δ=0.30. In a second step of themethod, the core rod is overclad by a jacket tube of undoped quartzglass in that said tube is collapsed onto the core rod. The complex ofcore rod and overcladding tube forms a quartz glass preform from whichthe single-mode fiber is subsequently drawn.

In this method the jacket material is provided in the form of anovercladding tube of quartz glass. In the case of synthetic quartz glassthe jacket tube is normally produced by the measure that a siliconcompound, such as SiCl₄, is oxidized or hydrolyzed with formation ofSiO₂ particles and the SiO₂ particles are deposited in layers onto acarrier rod. The carrier rod is subsequently removed, and the resultingtube of porous soot material is densely sintered.

It has been found that the known jacket tubes no longer satisfy theincreasing demands made on the optical qualities of optical fibers,particularly single-mode fibers, in an unrestricted way. Especially withthe increasing technical importance of single-mode fibers having verylow OH contents (attenuation at 1385 nm<0.34 dB/km), the jacket materialgains more and more importance with respect to the optical properties.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a jacket tube whichcan be produced at low costs on the one hand and which can be used forproducing optical fibers with low optical attenuation on the other hand.

Furthermore, it is the object of the present invention to provide anoptical fiber which can be produced at low costs and which has apredetermined attenuation portion in a reproducible manner in the rangeof the absorptions caused by OH groups. As for the jacket tube, thisobject, starting from the above-mentioned jacket tube, is achievedaccording to the invention in that the quartz glass of the jacket tubehas a content of metastable OH groups of less than 0.05 wt ppm and acontent of anneal-stable OH groups of at least 0.05 wt ppm.

A jacket tube within the meaning of this invention is a tube of quartzglass that is used for cladding a so-called core rod.

The OH group content (hydroxyl group content) of synthetically producedquartz glass is composed of chemically firmly bound OH groups whichcannot be removed by annealing the quartz glass, and of chemically lessfirmly bound OH groups which can be “annealed out” of the quartz glassby way of an annealing treatment. The last-mentioned species of hydroxylgroups will be called “metastable OH groups” in the following, and thefirst-mentioned species will be called “anneal-stable OH groups”.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the guided light intensity (Pouter/P0) in responseto the fiber radius.

DETAILED DESCRIPTION OF THE INVENTION

The jacket tube of the invention is optimized with respect to the twospecies of OH groups, which will be explained in more detail in thefollowing:

1. It has been found that a certain content of OH groups can reduce thetransportation of impurities in the quartz glass (specifically bydiffusion during the fiber drawing process). This result is surprisingbecause hydroxyl-containing quartz glass has a lower viscosity incomparison with hydroxyl-free quartz glass, which normally facilitatesthe diffusion of impurities into quartz glass at high temperatures.

However, it has also been found that this diffusion-inhibiting effect ofthe OH groups cannot definitely be correlated with the total hydroxylgroup content of the quartz glass. It has been found that only thechemically firmly bound, anneal-stable OH groups efficiently prevent thetransportation of impurities into quartz glass, whereas the metastableOH groups are inefficient in this respect. Possibly, this advantageouseffect of the anneal-stable OH groups is due to the fact that these arecapable of retaining the impurities in the jacket tube or inside theouter cladding region of the fiber due to chemical bonding or thatexisting defects or defects formed during fiber drawing are saturated byhydrogen or OH groups and are thus no longer available for a transportmechanism for impurities.

The jacket tube of the invention therefore consists of synthetic quartzglass having a concentration of anneal-stable OH groups of at least 0.05wt ppm. Within the meaning of this invention, the content ofanneal-stable OH groups is defined as the OH content that after heatingof a component with a thickness of not more than 10 mm remains in thequartz glass component (diffusion length≦5 mm) if heating is carried outat a temperature of 1040° C. for a period of 48 hours and with inert gasflushing.

Anneal-stable OH groups can only be removed from quartz glass by takinggreat technical efforts. A content of anneal-stable OH groups of 0.05 wtppm or more is adjustable with reasonable efforts.

The inventive jacket tube of synthetic quartz glass is thuscharacterized in that in the temperature and deformation processestypical of fiber drawing said tube only discharges a small amount ofimpurities towards the optical cladding and the core of the fiber andthat it can be produced at low costs.

2. When the fiber design is known, it is possible to calculate theinfluence of OH groups on optical attenuation, the OH groups beingcontained in the cladding region of the fiber. As a rule, the intensityof the radiation guided in the optical fiber decreases exponentiallyfrom the fiber core to the outside in response to the fiber design. Theamount of the guided light intensity (P_(outer)/P₀) in response to thefiber radius (standardized to the d_(M)/d_(K) ratio) can be inferredfrom the diagram of FIG. 1 for a standard single-mode fiber. As can beseen, the guided light intensity rapidly decreases with an increasingdistance from the core-cladding interface (d_(M)/D_(K) ratio=1). Forinstance, the amount of the light intensity at a distance from the fibercore that corresponds to a d_(M)/d_(K) ratio of 2.5 is only about 1/1000of the light intensity in the fiber center (d_(M)/d_(K) ratio=0).

On the basis of the formula

${\alpha_{1385}\left\lbrack {{dB}\text{/}{km}} \right\rbrack} \leq {\frac{P_{outer}\left( r_{0} \right)}{P_{0}} \times 62.7\mspace{14mu}\frac{dB}{{km} \cdot {ppm}} \times {{conc}._{OH}\left( {r > r_{0}} \right)}}$it is possible to determine the attenuation amount (α₁₃₈₅) at awavelength of 1385 nm from the intensity ratio at each point of thecladding region (P_(outer)(r₀)/P₀), the attenuation amount followingfrom a known concentration of OH groups (conc._(OH)) in the respectivecladding region (r>r₀). On the basis of this calculation the stillacceptable OH content of the respective cladding region has to bedetermined. It is a standard procedure among the experts to calculatethe still acceptable OH content in the cladding on the basis of theintensity profile known for the respective fiber and in consideration ofthe demand made on the quality of the optical fiber, and to produce acorresponding quartz glass which has an OH content with or below saidstill acceptable maximum content.

Surprisingly, however, it has been found that OH groups in the jackettube may have an adverse effect on the attenuation of the optical fiberbeyond the theoretically calculated contribution. Even at OH contentsclearly below the arithmetically determined and still acceptable maximumcontent of OH groups, excessive OH group absorptions occurred.Furthermore, it was found that this effect, too, cannot definitely becorrelated with the total hydroxyl group content of the quartz glass andis also hardly reproducible. It was found that this low reproducibilityis correlated with the chemically insignificantly bound metastable OHgroups. A possible explanation for the fact that the theoreticalcalculation of the influence of the OH groups on the optical fiberattenuation may lead to wrong results is that metastable OH groupsdiffuse due to their mobility during a hot treatment into regions closerto the core where they contribute, on account of the higher intensity ofthe guided light, to a much stronger absorption than in the claddingregion which is remote from the core and from which they derive. In thisrespect the fiber drawing process is particularly critical because attemperatures around 2000° C., which are typical of fiber drawing, thediffusion paths existing in a fiber with respect to the core are short,e.g. in a single-mode fiber they are less than 62 μm. Evidently, themetastable OH groups can easily bridge said paths, whereas anneal-stableOH groups are harmless in this respect.

The present invention is thus based on the finding that the stillacceptable attenuation portion by OH group absorption must only beoccupied by anneal-stable OH groups in the jacket material, but not bymetastable OH groups. The still acceptable attenuation portion doestherefore not define the upper limit for the total hydroxyl groupcontent in the jacket tube (as has so far been the case), but accordingto the invention the upper limit for the content of anneal-stable OHgroups in the jacket material, whereas the content of metastable OHgroups is ideally zero.

Therefore, the inventive jacket tube consists of synthetic quartz glasshaving a concentration of metastable OH groups of not more than 0.05 wtppm. Within the meaning of this invention the content of metastable OHgroups is defined as that OH content that is expelled out of a componentmade of the quartz glass with a thickness of not more than 10 mm byheating to a temperature of 1040° C. for a period of 48 hours with inertgas flushing.

Metastable OH groups can be expelled by an annealing process, asindicated in the above definition, relatively easily from the quartzglass. To keep the content of metastable OH groups in the quartz glassas small as possible, the presence and incorporation of metastable OHgroups may also be avoided or suppressed in a preventive way during themanufacturing process of the jacket tube. On the assumption that theformation of metastable OH groups is accompanied by the offer ofhydrogen or hydrogen-containing compounds during the manufacturingprocess of the jacket tube, a suitable possibility consists, forinstance, in largely avoiding hydrogen or hydrogen-containing compounds,especially in hot processes which the quartz glass is subjected to inthe course of the manufacturing process. Other possibilities of activelyeliminating metastable OH groups from quartz glass follow from thechemical procedure during the manufacturing process. Drying processesusing gaseous drying agents (halogens) should here be mentioned by wayof example, such processes reducing not only the content of metastableOH groups, but also the content of anneal-stable OH groups.

It is essential that the content of metastable OH groups in the quartzglass of the jacket tube can be kept low or set to a small value in arelatively inexpensive way. Since it is only the metastable OH contentthat poses problems with respect to the diffusion of the OH groups, theinventive jacket tube of synthetic quartz glass is thus alsocharacterized in that despite the inexpensive production in thetemperature and deformation processes typical of fiber drawing itdischarges only a little amount of OH groups towards the opticalcladding and the core of the fiber.

It has turned out to be particularly advantageous when the quartz glasshas a content of metastable OH groups of less than 0.01 wt ppm. Theprobability that OH groups pass into the optically particularly relevantregion of the fiber and effect absorption at said place decreases with adecreasing content of metastable OH groups.

As has already been explained at the outset, the content ofanneal-stable OH groups is set to be as high as possible to profit, onthe one hand, from the more economic way of production and from thepositive effect on the transportation of impurities of an OH-rich quartzglass. On the other hand, anneal-stable OH groups in the cladding regionof the fiber also contribute to an absorption of the guided light wave.As a consequence, this yields an upper limit for the content ofanneal-stable OH groups that is defined by the attenuation portioncaused by the OH groups in the optical fiber. The intensity of theradiation guided in the fiber decreases exponentially to the outside inthe cladding region. The further the quartz glass provided by the jackettube is away from the fiber core, the higher is the still acceptable OHcontent. A measure of this distance of the jacket tube material from thefiber core is the so-called d_(M)/d_(K) ratio. The smaller this ratio isthe closer will the jacket tube move to the fiber core and the lower isthe still acceptable content of anneal-stable OH groups.

Depending on the demand made on the optical quality of the fiber and inresponse to the distance of the jacket tube material from the fibercore, the inventive jacket tube advantageously consists of quartz glasshaving a content of anneal-stable OH groups of not more than 5 wt ppm,preferably a content of anneal-stable OH groups of not more than 1 wtppm, and particularly preferably a content of anneal-stable OH groups ofnot more than 0.5 wt ppm.

Such a jacket tube can preferably be used at a d_(M)/d_(K) ratio in therange between 2.5 and 8 without the content of anneal-stable OH groupshaving an inadmissible effect on fiber attenuation. Metastable OH groupsdiffuse at the high fiber drawing temperatures in the quartz glass,thereby causing a certain attenuation portion. Apart from this,metastable OH groups in the cladding of the fiber have the same effectas anneal-stable OH groups in the cladding of the fiber with respect tothe absorption caused thereby. The above observations regarding themaximum content of anneal-stable OH groups in dependence upon thed_(M)/d_(K) ratio are thus equally applicable to the total content of OHgroups, namely of metastable OH groups and anneal-stable OH groups.

With respect to the above-described diffusion-inhibiting “getter effect”of the anneal-stable OH groups relative to the impurities existing inthe quartz glass, it has turned out to be useful when the quartz glassof the jacket tube has a content of anneal-stable OH groups of at least0.1 wt ppm, preferably at least 0.3 wt ppm.

The inventive jacket tube made of synthetically produced quartz glass iseither used for producing a preform therefrom, from which preform theoptical fiber is drawn, or it is collapsed in coaxial arrangement with aso-called “core rod” having a core glass enveloped by an inner claddingonto the rod during the fiber drawing process. The last-mentionedprocess is called “ODD method” (overclad during drawing) in theliterature. In the case of an application for producing a preform andalso in the case of an application in an ODD method, an inventive jackettube is used for producing the necessary material, or several jackettubes according to the invention are used for this purpose.

In a preferred embodiment, the jacket tube has a ratio of outer diameterto inner diameter ranging from 2 to 8, preferably from 4 to 6. Such ajacket tube is suited because of its wall thickness and the relativelysmall inner diameter to apply the whole necessary cladding material(outer cladding) of a fiber. The provision of the whole outer claddingin the form of a single jacket tube has mainly economic advantagesbecause the tube can be produced in one operation, and interfaces in thecladding region are avoided

As for the optical fiber, the above-indicated object starting from ageneric fiber is achieved according to the invention in that the secondcladding region consists of synthetic quartz glass which is obtained byelongating a jacket tube according to the invention.

The jacket tube according to the invention is preferably used forcladding a core rod having a core with a diameter d_(K) and a claddingwhich envelopes the core and has an outer diameter d_(M), with theproviso that the ratio of d_(M)/d_(K) is at least 2.5 and preferablyranges between 3 and 6. The jacket tube is here collapsed onto the corerod, or it is elongated in a coaxial arrangement with a core rod in anODD method directly into a strand or fiber.

The low metastable OH content of the jacket tube prevents the diffusionof mobile OH groups in near-core regions due to high temperatures duringcollapsing, elongation and particularly during fiber drawing, and anoptical attenuation is achieved in the range of the OH group absorptionand is reproducible within the predetermined specification.

Moreover, the content of anneal-stable OH groups reduces the diffusionof other impurities during the said hot processes and, in this respect,has even an advantageous effect on fiber attenuation.

The invention will now be explained in more detail with reference toembodiments.

EXAMPLE 1

Deposition Process

A porous soot body is produced by outside deposition using a standardOVD method without addition of a dopant. To this end soot particles aredeposited in layers on a carrier rotating about its longitudinal axis byreciprocating a number of parallel deposition burners, each of thedeposition burners being fed with SiCl₄, and SiCl₄ being hydrolyzed in aburner flame in the presence of oxygen into SiO₂.

Dehydration Treatment

After the deposition method has been completed and the carrier removed,a soot tube is obtained that is subjected to a dehydration treatment forremoving hydroxyl groups introduced due to the production process. Tothis end the soot tube is introduced in vertical orientation into adehydration furnace and is first treated at a temperature in the rangeof 800° C. to about 900° C. in a chlorine-containing atmosphere. Thetreatment lasts for six hours. This yields a hydroxyl groupconcentration of about 0.45 wt ppm.

Vitrification

The soot tube treated in this way is vitrified in a vitrificationfurnace at a temperature in the range of about 1350° C., resulting in ajacket tube with the desired wall thickness that invariably has ahomogeneous OH content of about 0.45 wt ppm over the radialcross-section.

Forming, Treating and Sample Production

The outer wall of the jacket tube produced in this way from syntheticquartz glass is coarse-ground by means of a peripheral grinder equippedwith a #80 grinding stone, whereby the predetermined desired outsidediameter is substantially obtained. The inner surface is polished bymeans of a honing machine equipped with a #80 grinding stone. The degreeof polish is continuously increased by the grinding stones beingexchanged, the final treatment being carried out with a #800 grindingstone.

The outside of the jacket tube is then ground by means of an NCperipheral grinder. After it has been made sure that the jacket tube isproduced with a wall thickness within a predetermined tolerance range, ameasurement sample is separated from the jacket tube, in the form of anannular disk having a thickness of 10 mm, on the basis of which thecontent of anneal-stable OH groups is then determined in the quartzglass. The jacket tube and the annular disk are then etched in ahydrofluoric acid-containing etching solution for a short period oftime.

The jacket tube has an outer diameter of 150 mm and an inner diameter of50 mm and a length of 2500 mm.

Annealing Treatment

For further reducing the content of metastable OH groups, the jackettube is subjected to an annealing treatment at a temperature of 1040° C.for a period of 200 hours with nitrogen flushing. With a known diffusioncoefficient of the metastable OH groups in the quartz glass, it would bepossible to arithmetically determine the content thereof in the jackettube after the annealing treatment. As will be explained in more detailin the following, the annular disk is used in the embodiment for thispurpose in that said annular disk is subjected to the same pretreatmentas the jacket tube.

Results of the Measurements of the OH Content

Subsequently, the OH contents are determined by spectroscopy in thejacket tube and in the measurement sample in that measurements are eachtime carried out over the whole wall thickness of about 50 mm. Themeasurement place in the jacket tube is here in the center with respectto the two tube ends. The measurement results are indicated in Table 1.

Since in terms of spectroscopy a distinction cannot be made betweenmetastable OH groups and anneal-stable OH groups, the result of themeasurement as taken shows the total content of OH groups in the jackettube and in the annular disk, each averaged over the wall thickness. Dueto the increased diffusion length, the jacket tube (with a diffusionpath of about 25 mm) has at 0.35 wt ppm a slightly higher total contentof OH groups than the annular disk (with a diffusion path of 5 mm).

By comparison, the OH content of the annular disk could be lowered to0.32 wt ppm. Since the annular disk after the anneal treatment accordingto the above definition of the metastable OH content (temperature=1040°C., treatment period>48 h, diffusion path<5 mm, inert gas flushing) nolonger contains any measurable content of metastable OH groups, themeasured OH content of 0.32 wt ppm must be completely present in theform of anneal-stable OH groups. Since anneal-stable OH groups cannot beeliminated by annealing, this means that the jacket tube also has a meancontent of anneal-stable OH groups of 0.32 wt ppm and that thedifference with respect to the total content of OH groups measured inthe jacket tube indicates the content of metastable OH groups stillpresent in the jacket tube, the content being thus at 0.03 wt ppm.

Hence, it has been found that with the dehydration treatment and with areasonable amount of energy and time spent in the quartz glass of thejacket tube, it is possible to adjust a total hydroxyl group contentwhich, on the one hand, has a content of anneal-stable OH groups that isnot too high, but adequate, and from which, on the other hand,metastable OH groups can be removed by an additional annealing treatmentto such an extent that an optical fiber with low attenuation can beproduced using the jacket tube.

Use of the Jacket Tube According to the Invention

The jacket tube according to the invention is used for producing anoptical fiber in that the jacket tube with an inner diameter of 50 mm iscollapsed during fiber drawing in an ODD method onto a core rod producedaccording to the MCVD method, which has a core having a diameter d_(K)and a cladding enveloping the core (including the substrate tubeportion) with an outer diameter d_(M), the ratio of d_(M)/d_(K) being4.0. The core rod is here inserted into the jacket tube and fixedtherein such that its central axis coincides with that of the jackettube. The two ends of the complex obtained in this way are connected toa dummy material of quartz glass, and the complex is introduced into avertically oriented and electrically heated fiber drawing furnace fromthe top side and softened zonewise, starting with the lower end, at atemperature around 2180° C., and a fiber with an outer diameter of 125μm is drawn from the softened area. A negative pressure ranging from 200mm to 1000 mmAq is maintained in the gap remaining between core rod andquartz glass cylinder.

The resulting optical fiber having a diameter of 125 μm is characterizedby an optical attenuation around 0.30 dB/km at a wavelength of 1.385 μm.

EXAMPLE 2 Comparative Example with Respect to Example 1

A porous soot body is produced by outside deposition according toExample 1; it is dehydrated, vitrified, formed and surface-treated. Theresulting jacket tube has an outer diameter of 150 mm and an innerdiameter of 50 mm and a length of 2500 mm. An annular sample is takenfrom the jacket tube in accordance with Example 1. The measurements ofthe OH content in the quartz glass of the jacket tube and in the annulardisk yield a homogenous OH content of about 0.45 wt ppm over the radialcross-section in each instance. The annular disk is then subjected tothe annealing treatment indicated in Example 1, but not the jacket tube.

Results of the Measurements of the OH Content

Due to the annealing treatment the OH content of the annular disk couldbe lowered to 0.32 wt ppm. Since the annular disk no longer contains ameasurable content of metastable OH groups after the annealing treatmentaccording to the above definition of the metastable OH content(temperature=1040° C., treatment duration>48 h, diffusion path≦5 mm,inert gas flushing), the measured OH content of 0.32 wt ppm must becompletely present in the form of anneal-stable OH groups. Sinceanneal-stable OH groups cannot be eliminated by annealing, thedifference with respect to the total hydroxyl group content prior toannealing (0.45 wt ppm) indicates the content of metastable OH groupsremoved by annealing. Hence, this value amounting to 0.13 wt ppmcorresponds to the content of metastable OH groups in the non-annealedjacket tube.

Use of the Jacket Tube

The jacket tube according to Example 2 is used in the same way as theone described above with reference to Example 1 as a semi-finishedproduct for producing an optical fiber by collapsing it in an ODD methodonto a core rod, which corresponds to that of Example 1. The residualcladding material is provided by the jacket tube. The resulting opticalfiber with a diameter of 125 μm has an optical attenuation of less than0.43 dB/km at a wavelength of 1.385 μm.

EXAMPLE 3

Deposition Process

A porous soot body is produced by flame hydrolysis of SiCl₄ withoutaddition of a dopant by means of the OVD method, as described withreference to Example 1. After the deposition method has been completed,the carrier rod is removed. A transparent jacket tube is produced fromthe resulting soot tube, which has a density of about 25% of the densityof quartz glass, namely on the basis of the method explained by way ofexample hereinafter.

Dehydration Method

The soot tube is subjected to a dehydration treatment for removinghydroxyl groups introduced by the manufacturing process. To this end thesoot tube is introduced in vertical orientation into a dehydrationfurnace and is first treated at a temperature of around 850° C. in achlorine-containing atmosphere. The treatment lasts for about six hours.Subsequently, the total concentration of hydroxyl groups in the soottube is less than 1 wt ppm.

Pretreatment

The soot body is then introduced into a vitrification furnace with avertically oriented longitudinal axis and is exposed to the openatmosphere—though for a short period of time only. The soot tube isthereby contaminated again with hydroxyl groups which can pass into thequartz glass and form metastable OH groups therein. To eliminatemetastable OH groups, the soot tube is subjected to a pretreatmentinside the vitrification furnace.

To this end the vitrification furnace is first flushed with nitrogen,the internal pressure of the furnace is reduced to 0.1 mbar, and thefurnace is then heated. The soot tube is supplied continuously from thetop to the bottom, starting with the lower end, to the heating element(length: 600 mm) at a feed rate of 10 mm/min. At a temperature of theheating element of 1200° C. a maximum temperature of about 1180° C. isobserved on the surface of the soot tube. The internal pressure insidethe vitrification furnace is kept at 0.1 mbar by continuous evacuation.

On account of this zonewise vacuum and temperature treatment of the soottube inside the vitrification furnace, a release of metastable OH groupsis achieved prior to the subsequent vitrification, which will beexplained in more detail in the following.

Vitrification

Vitrification is carried out directly following the describedpretreatment in the same vitrification furnace in that the soot tube isnow supplied in reverse order, i.e. starting with the upper end,continuously from the bottom to the top to the heating element at a feedrate of 10 mm/min and is heated therein zonewise. The temperature of theheating element is preset to 1600° C., whereby a maximum temperature ofabout 1580° C. is observed on the surface of the soot tube. The internalpressure inside the vitrification furnace is kept at 0.1 mbar duringvitrification by continuous evacuation. The jacket tube obtained byvitrification has an outer diameter of 180 mm, an inner diameter of 50mm, and a length of 2500 mm.Elongation

In a subsequent process step, the jacket tube is elongated in anelectrically heated furnace under an inert gas atmosphere at acontrolled internal pressure to an outer diameter of 90 mm and an innerdiameter of 30 mm. A suitable elongation method is e.g. described inDE-A 195 36 960. During elongation the jacket tube is subdivided intosuitable production lengths, in this instance, into partial lengths of 2m.

Results of the Measurements of the OH Content

Subsequently, the hydroxyl group content of the jacket tube isdetermined after elongation. To this end an annular sample (disk) istaken from one end of the tube and the OH content is measured byspectroscopy at nine measuring points (measuring distance=5 mm), eachbeing evenly distributed over the circumference of the sample. A mean OHcontent of 1.0 wt ppm was measured. This content is substantiallyidentical with the integrated OH content measured over the whole lengthof the jacket tube.

To determine the amount of metastable OH groups in the measured totalhydroxyl group content, the annular disk is subjected to an annealingtreatment as explained above with reference to Example 1. The subsequentmeasurement of the OH content yielded a difference over the value priorto annealing of 0.02 wt ppm, which approximately corresponds to theamount of metastable OH groups in the jacket tube (see Table 1, Example3).

Since the jacket tube is only heated for a short period of time duringelongation, which has hardly an effect on the OH content, the OHcontents determined in the elongated jacket tube are substantiallyidentical with those prior to elongation.

Use of the Jacket Tube According to the Invention

The jacket tube according to the invention is used for producing apreform for an optical fiber. To this end it is collapsed onto a corerod which has a ratio of d_(M)/d_(K) of 4.5. An optical fiber with adiameter of 125 μm is drawn from the resulting preform by way of astandard drawing method, the fiber being characterized by an opticalattenuation of less than 0.30 dB/km at a wavelength of 1.385 μm.

TABLE 1 Dehydration Anneal Outer- Inner- treatment PretreatmentMetastable stable Total Θ Θ Length t T t T OH-content OH-contentOH-content No. Sample [mm] [mm] [mm] atm. [h] [° C.] [h] [° C.] [wtppm][wtppm] [wtppm] 1 Jacket tube 150 50 2500 Cl₂ 6 800–900 200 1040 0.030.32 0.35 Annular disk 150 50  10 0.00 0.32 0.32 2 Jacket tube 150 502500 Cl₂ 6 800–900 — — 0.13 0.32 0.45 Annular disk 150 50  10 200 10400.00 0.32 0.32 3 Jacket tube  90 30 2000 Cl₂ 6 850 1 1180 0.02 0.98 1.00(180) (50) (2500) Annular disk  90 30  10 Cl₂ 6 850 1 1180 0.00 0.980.98 Outer Θ = outer diameter; inner Θ = inner diameter; atm. =atmosphere during dehydration treatment; t = duration of the treatment;T = temperature during treatment

1. A jacket tube comprising synthetically produced quartz glass, saidjacket tube being made as a semi-finished product configured to be usedfor producing an outer cladding glass layer of an optical fiber, whereinthe quartz glass has a content of metastable OH groups of less than 0.05wt ppm and a content of anneal-stable OH groups of at least 0.05 wt ppm.2. The jacket tube according to claim 1, wherein the quartz glass has acontent of metastable OH groups of less than 0.01 wt ppm.
 3. The jackettube according to claim 1, wherein the quartz glass has a content ofanneal-stable OH groups of not more than 5 wt ppm.
 4. The jacket tubeaccording to claim 3, wherein the quartz glass has a content ofanneal-stable OH groups of not more than 1 wt ppm.
 5. The jacket tubeaccording to claim 3, wherein the quartz glass has a content ofanneal-stable OH groups of not more than 0.5 wt ppm.
 6. The jacket tubeaccording to claim 1, wherein the quartz glass has a content ofanneal-stable OH groups of at least 0.1 wt ppm.
 7. The jacket tubeaccording to claim 6, wherein the quartz glass has a content ofanneal-stable OH groups of at least 0.3 wt ppm.
 8. The jacket tubeaccording to claim 1, wherein it has a ratio of outer diameter to innerdiameter in the range between 2 and 8, preferably between 4 and
 6. 9. Anoptical fiber comprising a core having a diameter d_(K) and a firstcladding region cladding the core and having an outer diameter d_(M),and a second cladding region cladding the first cladding region, theratio of d_(M)/d_(K) being at least 2.5, wherein the second claddingregion consists of synthetic quartz glass obtained by elongation of ajacket tube according to claim 1.